This invention relates to a novel system to ensure that papaya are free of fruit flies so as to meet quarantine restrictions. The method includes multiple heated fluid treatments in combination with fruit selection based on color.
Papaya (Carica papaya L.) is a tropical plant, the fruit of which is ovoid-oblong to nearly spherical with an outer thin skin, a thick fleshy edible inner portion, and a central seed-containing cavity. The unripe fruit is smooth and green, turning yellowish or orange when ripe, with an orange or reddish-orange fleshy edible portion. The ripe edible flesh has the consistency of butter and a mild and pleasant flavor.
Papayas are the fourth most important source of farm income in Hawaii, exceeded only by sugar, pineapples, and macadamia nuts. It is estimated that the 1984 papaya production was about 7 million pounds with a value of $17 million. About 70 percent of the crop is shipped to the mainland United States and Canada, and 15 percent to Japan. The remaining 15 percent is consumed in Hawaii.
Tephritid fruit flies (Diptera: Tephritidae) such as the Mediterranean fruit fly [(Ceratitis capitata (Wiedemann)], the oriental fruit fly (Dacus dorsalis Hendel), and the melon fly (Dacus cucurbitae Coquillett) are present in Hawaii and have been shown to spread as larvae or eggs in fruits or vegetables shipped from infested areas. Fruit flies are among the most destructive insect pests of citrus, deciduous fruits, and vegetables; and papayas produced in Hawaii cannot be shipped to the mainland U.S. or Japanese markets without quarantine treatment due to possible fruit fly infestation. Because of the economic importance of the papaya crop to Hawaiian agriculture and the destructiveness of fruit flies, a procedure to disinfest the fruit of the flies is of vital importance.
In the past, the standard treatment for papayas was fumigation with ethylene dibromide (EDB). However, new quarantine procedures are needed as the United States Environmental Protection Agency cancelled the use of EDB as a food fumigant effective Sept. 1, 1984, because of concerns regarding its safety. Methyl bromide cannot be used on papayas because of the phytotoxicity at effective concentrations. The only other fumigant which has shown promise for papaya is phosphine, and this material requires a minimum treatment of 48 hours. One of the difficulties in developing quarantine treatments is the requirement that no insects survive to provide a breading population capable of becoming established at the destination. The widely accepted security level is "probit 9," or no more than 32 survivors per 1,000,000 treated insects at the 95% confidence level (mortality of 0.999968).
One suggested approach for disinfestation of fruit is the use of heat. A 15-minute, 50.degree. C. hot-water immersion treatment has been tested successfully for fruit fly disinfestation of bananas (Armstrong, Journal of Economic Entomology 75(5): 787-790 (1982)). While a 20-minute, 49.degree. C. hot water immersion has been used on papaya, to control postharvest decay, heat treatments alone, that is without additional treatment such as fumigation with EDB, have not been effective for disinfesting papaya. This is because papayas are so large and their thermal diffusivity so low that by the time the interior of the fruit is warm enough to kill eggs or larvae of the fruit flies, the exterior of the fruit is damaged. Thus, while the obvious approach to disinfest papayas using heat would be to heat the fruit longer or at higher temperatures than for decay control, damage to the papaya from thermal stress occurs before a probit 9 mortality is achieved. Relative to the standard decay control treatment, if either the submersion time or the water temperature is increased, scalding will result as manifested by the development of an undesirable splotchy off-color, ranging from bright orange to a leathery brown. Additionally, the subsequent coloring rate will be delayed (further increases will stop the yellowing process) and the natural aroma and flavor of papaya will be detrimentally affected (Hundtoft and Akamine, Journal of Agricultural Engineering Research 16:343-352 (1971)).
Seo et al. (Journal of Economic Entomology 76(3):535-538 (1983)) showed that infestation of papaya by the oriental fruit fly (the fruit fly variety which most often infests papaya) was related to fruit ripeness, and that fruit which are one-quarter or less ripe have minimal infestation. While such knowledge can be of assistance in selecting papaya with minimal fruit fly infestation, it is not sufficient for quarantine control, either alone or in combination with the heat treatment for decay control described above, because the required insect mortality cannot be achieved with sufficient certainty.